Enabling Higher-Order Modulation for Underwater Backscatter Communication

Sayed Saad Afzal, Reza Ghaffarivardavagh, Waleed Akbar, Osvy Rodriguez, and Fadel Adib. 2020. Enabling Higher-Order Modulationfor Underwater Backscatter Communication. Global OCEANS 2020.


Piezo-acoustic bacskcatter (PAB) is a recently-introduced ultra-low-power underwater communication technology. In contrast to traditional underwater acoustic modems,which need to generate power-consuming carrier signals, PAB nodes communicate by simply reflecting (i.e., backscattering) existing acoustic signals in the environment. This reflection-based approach enables them to communicate at net-zero power but also imposes significant constraints on their throughput and modulation schemes.We presentPAB-QAM, the first underwater backscatter design capable of achieving higher-order modulation. PAB-QAM exploits the electro-mechanical coupling property of piezoelectric transducers to modulate their reflection coefficients. Specifically, by strategically employing reactive circuit components (inductors), we demonstrate how PAB-QAM nodes can modulate the phase and amplitude of acoustic reflections and realize higher-order and spectrally-efficient modulation schemes such as QAM. We designed and built a prototype of PAB-QAM and empirically evaluated it underwater. Our empirical evaluation demonstrates that PAB-QAM can double the throughput of underwater backscatter without requiring additional power, spectrum, or cost. Looking ahead, such increased throughput paves way for various sub-sea IoT applications in ocean exploration, underwater climate monitoring, and marine life sensing.